1. Field of Invention
This invention relates to flight controls for helicopters and more particularly to such controls for helicopters which have dual, counterrotating, coaxial rotors so as to provide optimum performance and controllability.
2. Description of the Prior Art
While it has been known for some time that there is advantage to be gained by using dual, counterrotating, coaxial, rigid rotors as the lift generating mechanism in a helicopter, certain problems including control problems are involved such that the development of a practical helicopter utilizing such a rotor system is a recent development.
It is interesting to note that when the famed aviation pioneer, Igor I. Sikorsky, built his first helicopter early in the twentieth century, it included a dual, coaxial, counterrotating, rigid motor. Glauert suggested in his book "Aerodynamic Theory" that rotor roll moment might be overcome by using two, counterrotating, rigid rotors. Bergquist, Michel and Fradenburg advanced the art in their U.S. Pat. No. 3,409,248 when they suggested that differential lateral cyclic pitch be varied selectively as a function of aircraft forward speed to both cancel or reduce the roll moments and to optimally position the lift vector of each rotor so as to produce optimum lift-to-drag ratio performance. The mechanism taught in the Bergquist et al patent to accomplish this function was a simple linkage to provide an input directly to the control rods of each rotor either manually or through an air speed sensor which used a computer to derive correct gains. Lewis, in his U.S. Pat. No. 3,570,786, suggested the coupling of collective stick to the differential lateral cyclic inputs of the control system so as to produce differential lateral cyclic pitch as a function of collective stick input. Lewis felt that at high speed flight where the collective stick position is constant, that a constant differential lateral cyclic input would provide adequate efficiency.
Cheney recognized that the gyroscopic moments imposed upon rigid, counterrotating, coaxial rotors during maneuvers would produce blade stress and deflection in opposite directions, and taught in his U.S. Pat. No. 3,521,971 that a large fixed phase angle be used with each rotor. to introduce differential cyclic pitch to produce an aerodynamic moment to cancel the gyroscopic precession moments during these flight maneuvers.
It will therefore be seen that Bergquist, Michel and Fradenburg taught that differential lateral cyclic pitch in rigid, coaxial, counterrotating rotors should be introduced as a function of aircraft forward speed to produce optimum lift-to-drag ratio performance of the rotor in a steady state by selectively positioning the rotor lift vectors. Bergquist, Michel and Fradenburg did not address themselves to the cancellation of gyroscopic precession moments. Cheney taught the use of a large fixed phase angle in dual, counterrotating, coaxial, rigid rotors to cancel the gyroscopic precession moments created during maneuvers. We have come to the realization that Cheney's large fixed phase angle with cancel precession moments at one flight speed only and we now teach a control herein for a dual, counterrotating, coaxial, rigid rotor which will automatically optimally position the lift vectors on the rotors so as to produce optimum lift-to-drag ratio including cancellation of roll moments by these opposing lift vectors, and automatically cancel or minimize the gyroscopic precession moments at all flight speeds to thereby provide optimum helicopter performance and controllability in both steady state and maneuver mode of operation.